The invention relates to a process for producing anhydrous hydrogen chloride by the distillation of an aqueous hydrogen chloride solution.
As is well known, an aqueous hydrogen chloride solution forms in the course of distillation an azeotropic composition of a certain concentration at the distillation pressure. It is thought that the temperature of the azeotropic point increases and the hydrogen chloride concentration in the solution at the azeotropic point decreases as the pressure increases from a reduced pressure to a pressure near atmospheric pressure.
Under such circumstances, a number of liquid-gas equilibrium data for hydrogen chloride-water binary systems are found in the vicinity of the azeotropic points at pressures near atomospheric pressure, as represented by Sako, T., Hakuta, T., Yoshitome, H., J.Chem. Eng. Jpn., vol. 17, 381(1984). However, very few liquid-gas equilibrium data are found except in the vicinity of the azeotropic points.
Among literatures disclosing gas-liquid equilibrium data of the high pressure systems are Kao, J. T. F., J. Chem. Eng. Data, vol. 15, No. 3,362(1970) and Staple, B. G., Procopio Jr., J. M., Chem. Eng., November 16, 113(1970).
The former discloses data in which the highest hydrogen chloride concentration is 44.6 wt. %, the highest temperature is 70xc2x0 C., and the highest pressure is 15 bar A (absolute). The latter discloses data in which the highest hydrogen chloride concentration is 35 wt. %, the highest temperature is 230xc2x0 C., and the highest pressure is 7 MPa A.
Now, on the basis of the foregoing literatures, it has been proposed to recover a highly concentrated aqueous hydrogen chloride solution and produce anhydrous hydrogen chloride through distillation from crude hydrogen chloride generated as byproducts in many chemical processes and from crude hydrogen chloride obtained by the thermal decomposition of waste liquors containing organic chlorides formed in the production process of vinyl chloride resins.
Examples thereof include a process combining a vacuum distillation with a pressure distillation disclosed in Japanese Patent Publication No. 21318/1975, a process combining an adiabatic absorption with a distillation disclosed in Japanese Patent Laid-Open No. 67504/1980, a process for producing anhydrous hydrogen chloride disclosed in Japanese Patent Laid-Open No. 86901/1997, and the lice.
The above-mentioned compounds containing chlorine are burned in an incinerator, and the resultant waste gas containing hydrogen chloride is recovered in a waste gas boiler and treated as a gas at about 350xc2x0 C. (referred to as a feed gas hereunder).
FIG. 5 is a flowsheet showing an embodiment of a conventional process. Using FIG. 5, a process disclosed in Japanese Patent Publication No. 21318/1975 is illustrated.
In FIG. 5, the aforementioned feed gas containing hydrogen chloride and the bottom solution of an absorption column 1 are supplied to a quencher 50 through lines 210 and 201, respectively. In the quencher 50, hydrogen chloride in the feed gas containing the hydrogen chloride is partially absorbed in the bottom solution of the absorption column 1, and the resultant absorbed solution is supplied to a distillation column 2 through lines 202 and 11. At this time, the absorbed solution through the line 202 contains iron ions and the like so that part of the absorbed solution is periodically taken out through a line 203.
The feed gas from which the hydrogen chloride is partially removed is introduced through a line 8 into the absorption column 1, and water or a dilute aqueous hydrogen chloride solution is fed through a line 9 provided at the upper part of the absorption column 1. The liquid is brought into contact with the gas in the column by way of packings, plates or trays (not shown) to absorb the remaining hydrogen chloride. The crude aqueous hydrogen chloride solution collecting in the bottom of the absorption column 1 is sent to the distillation column 2 through lines 201, 202 and 11. The feed gas from which the remaining hydrogen chloride is removed is passed through a line 10 to be treated separately.
In the distillation column 2, distilled vapor from the top of the column is fed to a receiver 5 through a line 12 and condenser 4, and the condensate is refluxed through a line 13. Gaseous hydrogen chloride is sent at atmospheric pressure xe2x88x920.2 MPa G (gauge) through a line 14 to a compressor 250 where it is pressurized to 0.4-0.7 MPa G to be converted to pressurized anhydrous hydrogen chloride, which is used separately as feed materials for other processes.
The process according to the above-described publication is composed of an absorption column and a distillation column, and hence it is simple process wise. However, in view of the relationship between the operating pressure in the distillation column and the operating temperature at the bottom of the column, the operating pressure has to be limited to atmospheric pressure xe2x88x920.2 MPa G so that the operating temperature may not exceed a temperature against which the material of the distillation column can stand.
As-described above, processes using anhydrous hydrogen chloride as a feed material, such as vinyl chloride production processes, need an operating pressure of 0.4-0.7 MPa G. Therefore, it has been necessary to supply the feed material by increasing its pressure to the operating pressure by means of a compressor 250. In this case, however, the compressor 250 has suffered damage of corrosion.
The process for producing anhydrous hydrogen chloride disclosed in Japanese Patent Laid-Open No. 86901/1997 uses two distillation columns and comprises: a first step in which a feed material of an aqueous hydrogen chloride solution with a concentration above the hydrogen chloride concentration at the azeotropic point of water and hydrogen chloride under the operating pressure is fed to a first distillation column to distill it at a pressure of from atmospheric pressure to 0.2 MPa G so as to obtain a dilute aqueous hydrogen chloride solution of the azeotropic composition at the bottom and highly concentrated gaseous hydrogen chloride at the top; a second step in which an aqueous hydrogen chloride solution formed by condensing the highly concentrated gaseous hydrogen chloride obtained in the first distillation column is fed to a second distillation column to distill it at a pressure exceeding 0.2 MPa G and a temperature of 160xc2x0 C. or below so as to obtain anhydrous hydrogen chloride at the top and an aqueous hydrogen chloride solution with a concentration somewhat higher than the azeotropic composition corresponding to the aforesaid pressure at the bottom; and a third step in which the aqueous hydrogen chloride solution obtained at the bottom of the second distillation column is fed to the first distillation column. The publication describes that it is preferable to use steel or the like with a fluoroplastic lining applied as the material of the first distillation column and an impermeable graphite material as the material of the second distillation column. However, since it is not realistic from industrial viewpoints to use the expensive impermeable graphite material, it has been demanded not to use the material.
As described above, it has been expected to develop a process for producing anhydrous hydrogen chloride which permits the temperature at the bottom of the distillation column to remain in the range of temperatures at which the cheap materials formerly employed can be used, even if the operating pressure of the distillation column is increased to 0.4-0.7 MPa G or higher.
It is an object of the present invention to provide a process for producing anhydrous hydrogen chloride which permits the temperature at the bottom of the distillation column to remain in the range of temperatures at which the formerly employed cheap materials can be used, even if the operating pressure of the distillation column is increased to 0.4 MPa G or higher.
The present inventors have made intensive investigations to allow the bottom temperature to remain in the range of temperatures at which the formerly employed cheap materials can be used, even if the pressure of the distillation column is increased. As a result, such a salting-out effect has been observed that when an aqueous hydrogen chloride solution, to which a salt selected from neutral or basic alkaline earth metal chlorides or alkali metal chlorides is added, is distilled at atmospheric pressure, the temperature at the bottom of the distillation column is significantly reduced unexpectedly as compared with the case where no salt is added, in obtaining the same gaseous hydrogen chloride distillate. As a result of further intensive investigations, it has been found that the same salting-out effect is observed even under pressure.
The present invention has been completed on the basis of these findings, and the object of the invention is achieved by the means described below.
A process for producing anhydrous hydrogen chloride comprising subjecting an aqueous hydrogen chloride solution to distillation, wherein the distillation is carried out in the presence of a salt having a salting-out effect in the aqueous hydrogen chloride solution.
A process for producing anhydrous hydrogen chloride comprising bringing a feed gas comprising gaseous hydrogen chloride and a non-condensable gas into contact with at least one absorbing medium selected from water or a dilute aqueous hydrogen chloride solution in an absorption column to cause the absorbing medium to absorb the gaseous hydrogen chloride, and distilling the resultant absorbed solution in a distillation column to produce anhydrous hydrogen chloride, wherein the absorbed solution fed to the distillation column contains a salt having a salting-out effect.
A process for producing anhydrous hydrogen chloride comprising bringing a feed gas comprising gaseous hydrogen chloride and a non-condensable gas into contact with at least one absorbing medium selected from water or a dilute aqueous hydrogen chloride solution in an absorption column to cause the absorbing medium to absorb the gaseous hydrogen chloride, feeding the resultant absorbed solution dividedly to a first distillation column and to a condenser provided above the first distillation column, distilling in the first distillation column the absorbed solution fed thereto, bringing into the condenser the gas from the head of the first distillation column into contact with the absorbed solution fed to the condenser to cause the gas to condense, and distilling the resultant condensate in a second distillation column to produce anhydrous hydrogen chloride, wherein the absorbed solution fed to the first distillation column and the condenser contains a salt having a salting-out effect.